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<blockquote>
<p>先确定protected来自类A（有可能是基类，也有可能是object类，也有可能就是自己这个类），确定的时候注意最后一次被重写是什么时候，然后根据可见性（A类所在的包以及A类的子类）来判断</p>
</blockquote>
<h1 id="java-protected关键字详解">JAVA protected关键字详解</h1>
<p>很多介绍Java语言的书籍(包括《Java编程思想》)都对protected介绍的比较的简单，基本都是一句话，就是: <strong>被 protected 修饰的成员对于本包和其子类可见</strong>。这种说法有点太过含糊，常常会对大家造成误解。实际上，protected的可见性在于两点：</p>
<blockquote>
<ul>
<li>基类的 protected 成员是包内可见的，并且对子类可见；</li>
<li>若子类与基类不在同一包中，那么在子类中，子类实例可以访问其从基类继承而来的protected方法，而不能访问基类实例的protected方法。</li>
</ul>
</blockquote>
<p>我们可以通过以下几个关于protected方法可见性的例子来进一步掌握protected关键字。<strong>在碰到涉及protected成员的调用时，首先要确定出该protected成员来自何方，其可见性范围是什么，然后就可以判断出当前用法是否可行了</strong>，看下面七个例子：</p>
<a id="more"></a>
<h2 id="实例1">实例1</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p1;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Father1</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">f</span><span class="params">()</span> </span>{} <span class="comment">// 父类Father1中的protected方法</span></span><br><span class="line">}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">package</span> p1;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Son1</span> <span class="keyword">extends</span> <span class="title">Father1</span> </span>{}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">package</span> p11;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Son11</span> <span class="keyword">extends</span> <span class="title">Father1</span></span>{}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">package</span> p1;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test1</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> Son1 son1 = <span class="keyword">new</span> Son1();</span><br><span class="line"> son1.f(); <span class="comment">// Compile OK ----（1）</span></span><br><span class="line"> son1.clone(); <span class="comment">// Compile Error ----（2）</span></span><br><span class="line"> </span><br><span class="line"> Son11 son = <span class="keyword">new</span> Son11(); </span><br><span class="line"> son11.f(); <span class="comment">// Compile OK ----（3）</span></span><br><span class="line"> son11.clone(); <span class="comment">// Compile Error ----（4）</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于上面的示例，首先看(1)(3)，其中的f()方法从类Father1继承而来，其可见性是包p1及其子类Son1和Son11，而由于<strong>调用f()方法的类Test1所在的包也是p1，因此（1）(3)处编译通过</strong>。其次看(2)(4)，其中的clone()方法的可见性是java.lang包及其所有子类，对于语句"son1.clone();"和"son11.clone();"，<strong>二者的clone()在类Son1、Son11中是可见的，但对Test1是不可见的</strong>，因此（2）(4)处编译不通过。</p>
<h2 id="实例2">实例2</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p2;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyObject2</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">protected</span> Object <span class="title">clone</span><span class="params">()</span> <span class="keyword">throws</span> CloneNotSupportedException</span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">super</span>.clone();</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">package</span> p22;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test2</span> <span class="keyword">extends</span> <span class="title">MyObject2</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span> </span>{</span><br><span class="line"> MyObject2 obj = <span class="keyword">new</span> MyObject2();</span><br><span class="line"> obj.clone(); <span class="comment">// Compile Error ----（1）</span></span><br><span class="line"> </span><br><span class="line"> Test2 tobj = <span class="keyword">new</span> Test2();</span><br><span class="line"> tobj.clone(); <span class="comment">// Complie OK ----（2）</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于(1)而言，clone()方法来自于类MyObject2本身（因为MyObject2重写了clone()方法），因此其可见性为包p2及MyObject2的子类，<strong>虽然Test2是MyObject2的子类，但在Test2中不能访问基类MyObject2的protected方法clone()</strong>，因此编译不通过;对于(2)而言，<strong>由于在Test2中访问的是其本身实例的从基类MyObject2继承来的的clone()，因此编译通过</strong>。</p>
<h2 id="实例3">实例3</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p3;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyObject3</span> <span class="keyword">extends</span> <span class="title">Test3</span> </span>{</span><br><span class="line"> <span class="comment">//因为没有重写clone()，所以clone来自于Test3</span></span><br><span class="line">}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">package</span> p33;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test3</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span> </span>{</span><br><span class="line"> MyObject3 obj = <span class="keyword">new</span> MyObject3();</span><br><span class="line"> obj.clone(); <span class="comment">// Compile OK ------（1）</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于(1)而言，clone()方法来自于类Test3，因此其可见性为包p33及其子类MyObject3，而（1）正是在p33的类Test3中调用，属于同一包，编译通过。</p>
<h2 id="实例4">实例4</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p4;</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyObject4</span> <span class="keyword">extends</span> <span class="title">Test4</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">protected</span> Object <span class="title">clone</span><span class="params">()</span> <span class="keyword">throws</span> CloneNotSupportedException </span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">super</span>.clone();</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">package</span> p44;</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test4</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String args[])</span> </span>{</span><br><span class="line"> MyObject4 obj = <span class="keyword">new</span> MyObject4();</span><br><span class="line"> obj.clone(); <span class="comment">// Compile Error -----（1）</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于(1)而言，clone()方法来自于类MyObject4，因此其可见性为包p4及其子类(此处没有子类)，而类Test4却在包p44中，因此不满足可见性，编译不通过。</p>
<h2 id="实例5">实例5</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p5;</span><br><span class="line"> </span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyObject5</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">protected</span> Object <span class="title">clone</span><span class="params">()</span> <span class="keyword">throws</span> CloneNotSupportedException</span>{</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">super</span>.clone();</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test5</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> CloneNotSupportedException </span>{</span><br><span class="line"> MyObject5 obj = <span class="keyword">new</span> MyObject5();</span><br><span class="line"> obj.clone(); <span class="comment">// Compile OK ----(1)</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于(1)而言，clone()方法来自于类MyObject5，因此其可见性为包p5及其子类(此处没有子类)，而类Test5也在包p5中，因此满足可见性，编译通过。</p>
<h2 id="实例6">实例6</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p6;</span><br><span class="line"> </span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyObject6</span> <span class="keyword">extends</span> <span class="title">Test6</span></span>{}</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test6</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> MyObject6 obj = <span class="keyword">new</span> MyObject6();</span><br><span class="line"> obj.clone(); <span class="comment">// Compile OK -------（1）</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于(1)而言，clone()方法来自于类Test6，因此其可见性为包p6及其子类MyObject6，而类Test6也在包p6中，因此满足可见性，编译通过。</p>
<h2 id="实例7">实例7</h2>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">package</span> p7;</span><br><span class="line"> </span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">MyObject7</span> <span class="keyword">extends</span> <span class="title">Test7</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> Test7 test = <span class="keyword">new</span> Test7();</span><br><span class="line"> test.clone(); <span class="comment">// Compile Error ----- (1)</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"> </span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test7</span> </span>{</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>对于(1)而言，clone()方法<strong>来自于类Object</strong>，因此该clone()方法<strong>可见性为包java.lang及其子类Test7</strong>，由于类MyObject7不在此范围内，因此不满足可见性，编译不通过。</p>
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<div class="post-toc motion-element"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#java-protected关键字详解"><span class="nav-number">1.</span> <span class="nav-text">JAVA protected关键字详解</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#实例1"><span class="nav-number">1.1.</span> <span class="nav-text">实例1</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#实例2"><span class="nav-number">1.2.</span> <span class="nav-text">实例2</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#实例3"><span class="nav-number">1.3.</span> <span class="nav-text">实例3</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#实例4"><span class="nav-number">1.4.</span> <span class="nav-text">实例4</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#实例5"><span class="nav-number">1.5.</span> <span class="nav-text">实例5</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#实例6"><span class="nav-number">1.6.</span> <span class="nav-text">实例6</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#实例7"><span class="nav-number">1.7.</span> <span class="nav-text">实例7</span></a></li></ol></li></ol></div>
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